Hyper Velocity Impact Test of Kibo's Debris Shield

Hyper Velocity Impact Test of Kibo's Debris Shield

A Hyper Velocity Impact Test was conducted for the debris shield of the Japanese Experiment Module Kibo's Pressurized Module from July 21 to September 29, 1999. Kibo is designed so that the probability of its wall not being penetrated due to impacts of space debris or meteoroids during its ten-year operation period (no-penetration probability) is more than 97 %. To confirm whether Kibo's wall was manufactured as designed, a series of tests were conducted to simulate debris impacting with high speed on the debris shield of Kibo's pressurized module wall.

Space Debris and Meteoroid

Space debris consists of objects currently circling the Earth after being launched and completing their duties. It includes used satellites, final stages of rockets and their components, as well as fragments created by collisions between those components.
Since the orbital velocity of Space debris is so high, even 1 cm fragments has the energy of a speeding car. In July 1996, a small satellite, CERISE, collided with a suitcase-size piece of debris and a part of the body was lost. These days there is a movement to consider this situation as a new space environment problem.
A meteoroid is a natural object floating in space. When it comes into the atmosphere it is called a shooting star, and if it reaches the ground it is called a meteorite.

Kibo's Debris Shield

Kibo's debris shield

The International Space Station (ISS) wall is designed to protect the ISS and its crew from space debris. A debris shield is adopted for this particular purpose.

A famous astronomer, Fred Whipple, devised this shield, so it is called a Whipple Shield. The idea is to attach a layer of thin aluminum plate outside the wall. When a space debris impacts this outer plate, the kinetic energy of the space debris is converted to heat. This heat creates a hole in the outer plate. However, the debris will evaporate or disintegrates to small particles consisting of gas, liquid and solids and will no longer be able to significantly damage the wall.

As shown in the figure on the right, a stuffed-shield is adopted on the wall of the front side of Kibo, covering an area of 150 degrees. A stuffed shield binds several kinds of textile layers similar to a bulletproof vest between itself and the wall. This is a reinforced Whipple shield.

Whipple shield
		Structure
Stuffing shield
		Structure

Hyper Velocity Impact Test

Test results of simulated space debris with diameter 11 mm that impacts at a velocity of 5.71 km.

Shield	Stuffing	Pressurized wall

During the series of tests, simulated space debris, aluminum spheres, with diameters of 5 mm, 7 mm, 9 mm, and 11 mm impacted the wall of Kibo at velocities of 2.5 to 6.0 km. In order to evaluate whether the ballistic limit curve is appropriate, we conducted the tests with a variety of combinations of debris diameters and velocities. The test results proved that the ballistic limit curve is appropriate, and Kibo' wall has the strength required.

A ballistic limit curve is a graph generated by theoretically computing the relation between size and velocity of the space debris which penetrates the wall. This test confirms that the ballistic limit curve is appropriate, not only theoretically but also practically, and that Kibo has the required strength as well. However, the projectile velocity generated by the test equipment is limited. Whether or not projectile will penetrate the wall at velocities beyond the capability of the test equipment, will be extrapolated from the ballistic limit curve.

Ballistic limit curve

The following space debris policy will be adopted for total ISS. For space debris with a diameter exceeding 10 cm, compute its orbit before hand and maneuver the ISS orbit to avoid a collision. If space debris with a diameter more than 1 cm but less than 10 cm penetrates the wall, astronauts will escape to an adjoining module and close the hatch. The hole will be repaired later by EVA. The debris shield will be used to prevent space debris with a diameter of less than 1 cm from penetrating the wall. Debris penetration will depend on the velocity and impact angle. As a result, a piece of debris exceeding 10 mm in diameter might not penetrate the wall, whereas a smaller piece might. International partners are now cooperating to improve observation ability from the ground so that debris smaller than 10 cm in diameter can be acquired.

Test tool Two-stage light gas-gun

A 21 m-long two-stage light gas gun facility was developed to fire a projectile into a test object. This facility was developed to generate projectile velocities of several kilometers per second, which is much faster than a pistol bullet.

Two-stage light gas-gun composition

Sabot and Simulated piece of space debris

Diaphragm

Piston

A simulated piece of space debris (aluminum balls with diameter of 3 thru 11 mm (approximately 0.04 thru 1.9 g)), is fired after being inserted into a plastic case called a "sabot". The aluminum ball can be accelerated efficiently by a highly pressurized gas. The top of the sabot is hollow, which facilitates its destruction by air resistance so the aluminum ball inside can escape.
When the gunpowder is ignited and the exploded gas pressure reaches a predetermined level, the diaphragm, a thin metal plate, located between the gunpowder chamber and the pump tube is broken, and the plastic piston is pushed out. The pump tube is filled with a light gas such as helium or hydrogen and pressurized to seven atmospheres. (note)
The piston pressurizes the light gas inside the pump tube. The pressure of the light gas breaks the diaphragm located between the high-pressure coupling and the launch tube. High-pressure gas then bursts into the launch tube and accelerates the sabot. The piston is stopped by the taper of the high-pressure coupling. The launch tube and the sample room are filled with 0.1 thru 0.2 atmosphere of nitrogen gas. The sabot containing the simulated space debris is broken and separated by the resistance of the nitrogen gas, and stopped by the iron plate sabot trap. The aluminum ball then leaves the sabot, proceeds straight forward, and impacts the sample.
The velocity measuring device, located just in front of the sabot trap, has two coils. The projectile velocity can be measured by measuring the voltage generated when the sabot with an embedded permanent magnet passes through the coil.

(note) Highly pressurized gas disturbs the movement of objects inside. Light gas like helium or hydrogen inside, allows easier movement of the simulated space debris.

*Click parts' names to view their images

		Test Procedure
		Test preparation and the test are conducted as shown below.

(1)	Disassemble	Disassemble the two-stage light gas gun.	Disassembled firing pipe

(2)	Cleaning	Remove soot caused by previous firing, and check for scratches inside.	Clean the disassembled firing pipe.

(3)	Assemble the light gas gun.	Assemble the cleaned up light gas gun. Change diaphragm and piston with new ones.	Changed new diaphragm

(4)	Check the launch tube.	The launch tube is composed of five parts. When they are assembled, the junctions should be smooth.	Check for smoothness

(5)	Speed sensor check	Check the sensor for measuring the speed of projectile to ensure it functions perfectly.

(6)	Attaching sabot trap	The sabot is a case containing the aluminum ball. A sabot trap catches the sabot which breaks apart in the launch tube.	Checking the simulated debris' path using a laser beam.

(7)	Setting the test object	Set the test object in the sample room. Adjust the position of the test object so that the simulated debris impacts at the center of the object.	The red spot at the center of the sample plate is the laser beam.
(8)	Fill the pump tube with light gas	Fill the pressure pipe with light gas, then pressurize it to seven atmospheres.

(9)	Fill nitrogen gas	Fill the space from the firing pipe to the target chamber with 0.1 thru 0.2 atmospheres of nitrogen.

(10)	Load gunpowder.	Load gunpowder into the gunpowder chamber. Adjust the volume of gunpowder according to light gas or speed of the projectile.

(11)	Firing	Insert an igniter into the gunpowder to complete preparation. Move safely away and then fire.

Since the above preparations take three to four hours, a maximum of only two tests is possible per day.

	Interviews with key persons
		NASDA personnel's comments

		During the early test period, we could not start the test because we had difficulty measuring the speed of the simulated debris. When I finally resolved the problem in cooperation with experts, I felt quite relieved. The debris velocity is a very important factor in this test, so we were very concerned about velocity measurement. In the latter part of the test period, we eagerly sought to obtain effective data. Based upon the test results of the day, we were anxious to find the best conditions for the tests of the next day.
	Kaoru Miyake JEM project team Office of Space Utilization Systems NASDA

Interviewing Mitsubishi Heavy Industry personnel


Mr. Hiroaki Takahashi	Mr. Katsuyuki Enomoto	Mr. Tomikazu Kusumoto

We interviewed Mitsubishi Heavy Industry's Mr. Takahashi, Mr. Enomoto, and Mr. Kusumoto who were responsible for conducting the tests.

‚pjWhat are the most difficult points of this test?
‚`jThe test itself doesn't take time. However, preparation takes a long time, so we can conduct tests only twice a day, at most.

‚pjFor what do you pay most attention?
‚`jWe have to be most careful in keeping the firing pipe perfectly straight. If the firing pipe is not straight, the projectile doesn't fly under perfect conditions. When assembling the five parts of the firing pipe, we need to be very careful. The junctions of the pipe have eight bolts, and if even only one of them is not fastened appropriately, the firing pipe can't be kept straight. Recovering from such a situation is a messy task.

Last Updated : September 18,2003